Side thrust compensation for hydraulic units



Nov. 9, 1965 H. THOMA SIDE THRUST COMPENSATION FOR HYDRAULIC UNITS Filed Jan. 22, 1964 F I NVENTOR H us Thom Patented Nov. 9, 1965 3,216,333 SIDE THRUST COMPENSATION FOR HYDRAULIC UNITS Hans Thoma, Bellevueweg 25, Zug, Switzerland Filed Jan. 22, 1964, Ser. No. 339,522 Claims priority, application Switzerland, Jan. 25, 1963, 833/63; Germany, Jan. 8, 1964, T 25,391 Claims. (Cl. 92l10) The present invention relates to hydraulic units such as hydraulic pumps or motors of the reciprocating ram or piston type and, more specifically, is directed to an improved arrangement for the compensation of side-thrust exerted on each ram or piston.

It is already known to the art to provide compensating means for the side-thrust of pistons transmitting torque to the cylinder barrel of the hydraulic unit, by utilizing the inclination of a slipper or shoe arranged at the outer end of the piston and operably connected with such piston by a suitable articulation, generally a ball-shaped articulation joint. Since such side-thrust to which the pistons are subjected is often very pronounced, it is known to provide on the surface of the piston sliding along the cylinder Wall large recesses or excavations. Suitable ducts connect these recesses with the sliding surfaces of the shoe or slipper articulation in order to provide every time a sufficient oil pressure field just at the location where the piston has to transmit the side-thrust to the cylinder wall. Such an arrangement is disclosed in United States Patent 1,274,391 to William M. Davis, granted August 6, 1918, and entitled Hydraulic Transmission Mechanism.

It has been found, however, that large recesses controlled or pressurized by the inclination of a slipper or shoe are not working properly since the increase of the pressure in the lange recesses requires a certain amount of time due to the fact that it is not possible to construct the channels for the infeed of oil to pressurize the recesses sufiiciently large to fill-up such recesses within only some thousands of a second, as would be necessary in high-speed units commonly employed nowadays. Furthermore, an even more important difficulty arises from the fact that the pressure in these recesses, clue to their relatively large dimensions, will still remain for a considerable period of time after the instance in which the inclination of the slipper controlling the infeed of oil has interrupted the fluid connection between the pressurized cylinder and each of the recesses. As a result, the oil pressure in these recesses will remain for a considerably long period of time, even when the side-thrust between the piston and the cylinder wall has already changed direction. Consequently, the pressurized recesses will remain under pressure even when a piston has changed over from the pressure side to the suction side of the hydraulic unit. The result of this is that, those pistons having changed over to the suction side are subjected to a very strong hydraulic side-thrust by the pressurized recesses, even though such pistons no longer have any pressure on the working surface and, thus, the sliding surfaces of the pistons are unduly loaded and damaged or worn by such incorrect side-thrust compensation. Of course, it is conceivable to provide only small recesses on the piston surface, but, in this instance, it is not possible to provide for a compensation of the sidethrust on the piston when such is disposed at the pressure side of the hydraulic unit.

It is, therefore, a primary object of the present invention to provide an improved construction of hydraulic unit, particularly hydraulic pumps or motors of the reciprocating piston type, wherein the side-thrust exerted upon the pistons can be adequately and positively compensated.

A further important object of this invention is the provision of an improved hydraulic unit which provides an ample area for relieving the oil or liquid pressure field,

yet avoids accumulation of larger quantities of liquid at such location.

A further noteworthy object of this invention is the provision of an improved side-thrust compensation means for hydraulic units which overcomes the aforementioned disadvantages of prior art apparatus of this type.

In order to implement such objects and to avoid the difiiculties of the prior art apparatus, the present invention contemplates providing on the sliding surfaces of the piston, and for the purpose of side-thrust compensation, instead of the known sufiiciently large recesses, only small grooves which are arranged at the sliding surface of the piston mainly in the longitudinal direction thereof, with such grooves being long enough to transmit the pressure appearing in each small groove over a sufliciently long portion of the piston surface. According to one manifestation of the invention these grooves may, for instance, partake the form of small scratches or rills as such have previously been used for continuously admitting oil when the piston is on the pressure side, as for example taught in United States Patent 2,300,009 to Rose, yet in such case the admission of oil is not controlled by the inclination of a slipper or shoe.

Now, in accordance with the present invention where, according to one desired embodiment, small scratches are provided for the distribution of pressurized oil and which are controlled by the inclination of a shoe or slipper, there will no longer appear any considerable amount of pressurized oil in these scratch-like grooves including the channels conducting the oil pressure from the slipper to the scratches. Nonetheless, these comparatively small scratches will provide a quick distribution of pressurized oil over a sufliciently large area of the piston surface, at least if the cross-section of these scratches is not too large in comparison with the crosssection of the aforementioned channels or oil ducts, since the velocity of a pressure wave attains a value of about 3000 feet per second. Therefore, the pressure is admitted to the neighborhood of a comparatively large region of the piston surface approximately without any delay, so that the pressure wave in the scratches restrains the squeezing out of the oil content in the oil film between the sliding surfaces in the neighborhood of these scratches. Even if, at some location or at some time, there no longer appears such an oil film, with the sliding surfaces being in what is ordinarily considered direct metallic contact, such a condition would not be crucial since every metallic surface, as is known, has at least a microscopic amount of surface roughness, so that the pressurized oil infeed will creep between the sliding surfaces and spread over a large portion of such surfaces which are considered to be in so-called direct metallic contact. To prove this point that as a practical matter there is not really direct metallic contact of the sliding surfaces even in the absence of the aforementioned oil film, it should be considered that even two precisely machined or polished metallic surfaces which are pressed together will be passed by drops of pressurized oil, unless, of course, a strip of rubber is inserted therebetween to block-up the smallest irregularities of the surfaces, an expedient commonly practised for example by the use of rubber strips or the Well known O-rings.

Thus, the small scratches, themselves already known in the art for constantly supplied surfaces of a piston surface which is not relieved by an oil stream controlled by the inclination of a shoe or slipper, if used for pressure relief of a sliding surface controlled by the slipper inclination according to the teachings of the invention, provide the same advantages as are present with the use of the large recesses already known to the art, yet without the attendant disadvantages of such large recesses. This means, in effect, there is avoided the slow rise of the pressure within the large recesses, and of even greater importance, the completely incorrect pressure relief of the piston when changed over to the suction side due to the presence of the remaining oil pressure appearing in the large recesses by virtue of the elasticity of the large oil content of a large recess as employed up to the present for known side-thrust relief devices controlled by the inclination of the slippers, as known to the prior art.

In accordance with another important modification of the present invention the grooves may be advantageously constructed in the form of flattened or substantially flattened surfaces. These so-called fiattenings, and as will be pointed out herein, may possess different configurations and, if desired, can also be combined with scratch-like grooves.

Other features, objects and advantages of the invention will become apparent by reference to the following detailed description and drawing in which:

FIGURE 1 is a fragmentary, sectional elevation of a portion of a hydraulic pump or motor having a swash plate type of drive means and illustrating details of the improved side-thrust compensation means for the therein illustrated piston member;

FIGURE 2 is an enlarged development showing details of one form of pressurized liquid distribution grooves constructed in the form of scratches or relatively narrow rills;

FIGURE 3 illustrates details of a piston provided with a modified form of grooves embodying flattenings for the hydraulicunit depicted in FIGURE 1; and

FIGURE 4 is an enlarged cross-sectional view of the piston unit shown in FIGURE 3, and taken along lines IVIV thereof.

Directing attention now to the drawings and, more specifically, to the embodiment of FIGURES 1 and 2, where it will be appreciated that only enough of the structure of the hydrostatic power unit has been shown, for convenience in illustration, to enable one skilled in the art to which the invention is addressed to understand the underlying principles thereof, it will be seen that there is provided a ram or piston 1 which reciprocates in the bore 1a of the cylinder block 2. When this hydraulic or hydrostatic unit operates as a pump the ram or piston 1 is reciprocated by contact with an inclined swash plate 5 through the agency of a shoe or slipper 4. While the hydraulic unit can operate with any suitable fluid, oil is generally employed.

In the present instance, the ram or piston 1 is provided at its outer or free end with a ball 3 which engages in a correspondingly shaped socket 4a provided at the shoe 4, so that such shoe is able to turn about or slide on the ball 3 and maintain a fluidtight seal therebetween. Quite obviously, it is possible to replace the ball head of the piston 1 of this embodiment, as well as all other embodiments herein disclosed, by a ball cup which surrounds a ball element connected to the slide shoe 4, such as disclosed in the previously mentioned Davis patent. In other words, it is immaterial whether the ball 3 is provided at the piston 1, as shown, or at the shoe 4.

The ram or piston 1 is advantageously drilled axially through its length to provide a central bore 1!). A pin 6 is pressed or inserted into this central bore 112 and extends towards the outer surface of the ball 3, terminating somewhat forwardly thereof. A screw 7 or similar fastening expedient is provided to retain the pin 6 secured within central bore 1b. The pin 6, in turn, is axially drilled to provide a central bore 10 which extends from one end to the other of this aforesaid pin 6.

As can be seerrfrom FIGURE 1, the end of the central bore 10 of pin 6 is open to pressure fluid or liquid entering through the central bore 1b of the piston 1 and coming from the cylinder chamber or bore 1a. This bore 10 communicates with a small recess or bore 11 leading to the spherical or ball engaging cavity 4a of the shoe 4. It will further be-seen that this-small-bore-ll d communicates with at least one inclined bore 12 extending to an annular groove 13 provided at the shoe 4, more specifically, at the marginal surface of the ball engaging cavity 4a thereof.

Furthermore, the ball 3 of the piston 1 is provided with a number of control holes 14, for example six in number, communicating with a corresponding number of lengthwise extending channels 8. Each of these channels 8, in turn, communicate with a radial channel 9 merging with an associated small distribution groove 15 provided at the outer sliding surfaces of the piston 1. These small distribution grooves 15 which, for example, in the present instance also correspond to six in number, are formed so as to be very small in width and depth, corresponding to what may be considered as scratches and posses a length which is at least equal, although generally larger, than the radius of the piston 1. Moreover, the cross-section or cross-sectional area of these scratches or small grooves 15 does not exceed by any great amount the cross-section of the ducts or channels conducting the pressure oil or liquid from the shoe or slipper 4 to the loaded sliding surfaces of the piston 1. Preferably, such cross-section of these scratches 15 is only about two or three times larger than that of the oil pressure conducting ducts aforementioned. On the other hand, the crosssection of such scratches 15 can be smaller than that of the pressure oil conducting channels or ducts which conduct oil from the slipper 4 to the loaded surfaces, in order to advantageously provide for throttling of the oil. Such scratches 15 can be of small cross-sectional area for throttling purposes Without fear of clogging by foreign material, due to the movement of the outer surface of pitton 1 on the associated cylinder wall, which would then tend to disintegrate or comminute such undersirable foreign material. Additionally, these scratches or small grooves 15 may be provided at their ends with small lateral or transverse grooves 15a (FIGURE 2) to provide for enhanced oil distribution on the sliding surfaces, yet co a structed and arranged such as to avoid a direct conne tion between adjacent small grooves 15 arranged about the outer sliding surfaces of the piston 1.

Consistent with the construction aforedescribed, it will be seen that if the shoe 4 assumes the inclined position relative to the ball 3 of piston 1, as shown in FIGURE 1, the control holes or bores 14 at the right side of the piston 1 will not communicate with the annular groove 13, due to the valving action of shoe 4 and ball 3, while the control holes 14 at the left side of this piston 1 will be in direct fluid communication with such annular groove 13. Pressure oil is thus free to flow up the central bore 10, through bore 11, the bore 12, the annular groove 13, the open control holes 14 and the associated lengthwise channels 8 and radial ducts 9 to the corresponding small grooves or scratches 15. As a result, such oil can arrive at the required pressure area between the outer surface of the piston 1 and its cylinder 2 where the highest surface pressure occurs due to torque transmission; in the present instance such being at the left-side of the piston 1 when the inclination of the shoe 4 is as shown in FIG- URE l. Quite obviously, and if required, the piston can be provided at still further locations with additional groups of such scratches or small grooves 15 which com municate with the lengthwise channels 8 by similar, further radial ducts 9. In the event such additional small grooves 15 are provided at the bottom end of the piston 1 then, of course, the pin 6 would be somewhat lengthened. Generally, however, it will be unnecessary to provide such a set of pressure areas at the bottom end of the piston.

Small grooves 15 of the type aforedescribed may also be formed in the shape of flattenings or replaced by such, since such a flattening is nothing more than a groove provided with a special cross-sectional configuration, and which is particularly advantageous when considering the fact that it can be easily produced on a grinding machine, even upon a hardened surface of' the piston, and is very accurate to check for correct dimensioning by means of a micrometer for example.

In FIGURES 3 and 4 wherein like reference numerals generally designate similar elements, there is shown such a modified form of piston 20 provided with flattened grooves or flattenings 21, shown somewhat exaggerated to facilitate illustration, and replacing the scratch-like grooves 15 of FIGURE 1 commonly used with a triangular, square or semi-circular cross-sectional configuration. The shape of the flattened grooves or flattenings 21 of this embodiment may be, for example, accurately plane or even somewhat curved so as to be concave or convex, depending upon the shape of the grinding or milling device employed, or even a file or other tools which can be conveniently used for producing such flattened grooves or flattenings. Here again, the cross-section and length of these flattenings is advantageously dimensioned as previously described.

Moreover, flattenings 21 and scratches 15 can be combined, if desired, by providing a small longitudinal groove within the flattening for instance, perhaps at the center of each such flattening. Furthermore, it is not essential to construct the distribution grooves in the form of scratches 15 and/or flattenings 21 along their length with absolutely constant cross-section, since in some cases a tapering of these along their length may be advantageous. In such case, the average cross-section of such modified form of distribution grooves should fulfill the conditions aforedescribed. In some instances, it is also advisable to combine flattenings 21 and scratch-like grooves 15 such that, for example, a scratch-like groove is interrupted at several locations by short flattenings 21, particularly since such a combination of these different type grooves favorably influences the oil streaming in such distribution grooves.

It should further be mentioned that flattened grooves or flattenings 21 have the feature of facilitating the creeping-in of pressurized oil between the sliding surfaces of the piston and cylinder, particularly if these surfaces are very close together, or if the oil film between these should be very thin. Essential for the invention is to provide the cross-section or cross-sectional area of the distribution grooves 15 and/or 21 so that at most they are not very much larger than the oil ducts extending from the outer end of the piston to such grooves, but sufliciently long for supporting the side-thrust exerted upon sufliciently large pressurized surfaces.

Finally, it is also possible to provide a cross head in known manner on the outer piston end in order to use its slide surface or the relieving thereof for the purpose according to the invention. It will be also recalled that as was mentioned herein, the force transmitting members or shoes are capable of modification.

While there is shown and described present preferred embodiments of the invention it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practised within the scope of the following claims.

What is claimed is:

1. A hydraulic unit including a piston having sliding surfaces and arranged for movement within a cylinder, a force transmitting member disposed for relative movement with respect to said displaceable piston in order to control infeed of a pressurized fluid to said sliding surfaces, an inclined swash plate for displacing said displaceable piston through the agency of said force transmitting member, said piston being subject to side-thrust loads during operation, said displaceable piston being provided with a plurality of small grooves arranged about the outer surface of said displaceable piston and extending in the lengthwise direction thereof, duct means communicating with said small grooves for conducting pressurized fluid from said force transmitting member to said sliding surfaces of the piston subject to side-thrust, said small grooves possessing a cross-section which does not consid- 6 erably exceed the cross-section of said duct means and a length at least equal to the length of the radius of said displaceable piston.

2. A hydraulic unit according to claim 1 wherein the length of said small grooves is larger than the radius of said displaceable piston.

3. A hydraulic unit according to claim 2 wherein the cross-section of said small grooves is larger than the crosssection of said duct means by a factor which does not exceed 3.

4. A hydraulic unit according to claim 2 wherein the cross-section of said small grooves is smaller than the cross-section of said duct means to provide for throttling of said pressurized fluid.

5. A hydraulic unit according to claim 2 wherein said small grooves are configured to define scratches.

6. A hydraulic unit according to claim 5 wherein each of said scratches are provided at their ends with small lateral grooves spaced from the lateral grooves of an adjacent scratch to prevent intercommunication of said scratches.

7. A hydraulic unit according to claim 1 wherein said small grooves are configured to define flattenings.

8. A hydraulic unit including a piston having sliding surfaces and arranged for movement within a cylinder, a shoe disposed for relative rocking movement with respect to said displaceable piston in order to control infeed of a pressurized fluid to said sliding surfaces, an inclined member for displacing said displaceable piston through the agency of said shoe, said piston being subject to sidethrust loads during operation, said displaceable piston being provided with a plurality of small grooves arranged in the region of said sliding surfaces about the outer surface of said displaceable piston and extending in the lengthwise direction thereof, duct means communicating with said small grooves for conducting pressurized fluid from said shoe to said sliding surfaces of the piston subject to side-thrust, said small grooves possessing a crosssection which does not considerably exceed the cross-section of said duct means and a length greater than the length of the radius of said displaceable piston.

9. A hydraulic unit including a piston having sliding surfaces and arranged for movement within a cylinder, a force transmitting member disposed for relative movement with respect to said displaceable piston in order to control infeed of a pressurized fluid to said sliding surfaces, an inclined swash plate for displacing said displaceable piston through the agency of said force transmitting member, said piston being subject to side-thrust loads during operation, said piston being provided with a plurality of small grooves in the form of flattenings arranged about the outer surface of said displaceable piston and extending in the lengthwise direction thereof, duct means communicating with said flattenings for conducting pressurized fluid from said force transmitting member to said sliding surfaces of the piston subject to side-thrust.

10. A piston for a hydraulic unit comprising a body member provided with a plurality of small grooves arranged about the outer surface of said body member and extending in the lengthwise direction thereof, duct means provided at said body member communicating with said small grooves, said small grooves possessing a cross-section which does not considerably exceed the cross-section of said duct means and a length at least equal to the length of the radius of said body member.

11. A piston for a hydraulic unit according to claim 10 wherein the cross-section of said small grooves is smaller than the cross-section of said duct means.

12. A piston for a hydraulic unit comprising a body member provided with a plurality of small grooves defining flattenings arranged about the outer surface of said body member and extending in the lengthwise direction thereof.

13. A piston for a hydraulic unit comprising a body member provided with a plurality of small grooves defining fiattenings arranged about the outer surface of said body member and extending in the lengthwise direction thereof, said flattenings possessing a length at least equal to the length of the radius of said body member.

14. A piston for a hydraulic unit according to claim 13 wherein the length of said fiattenings is greater than the length of the radius of said body member 15. A piston for a hydraulic unit comprising a body member provided with a plurality of small grooves arranged about the outer surface of said body member and extending in the lengthwise direction thereof, said small grooves being configured to define flattenings, ductmeans 8v provided at said body member communicating withsaid small grooves, said small grooves possessing a cross-section which does not considerably xceed the cross-section of said duct means and a length greater than the length of the radius of said body member.

References Cited by the Examiner UNITED STATES PATENTS 3,106,138 10/63 Thoma 92-156 KARL J. ALBRECHT, Primary Examiner. 

1. A HYDRAULIC UNIT INCLUDING PISTON HAVING SLIDING SURFACES AND ARRANGED FOR MOVEMENT WITHIN A CYLINDER, A FORCE TRANSMITTING MEMBER DISPOSED FOR RELATIVE MOVEMENT WITH RESPECT TO SAID DISPLACEABLE PISTON IN ORDER TO CONTROL INFEED OF A PRESSURIZED FLUID TO SAID SLIDING SURFACES, AN INCLINED SWASH PLATE FOR DISPLACING SAID DISPLACEABLE PISTON THROUGH THE AGENCY OF SAID FORCE TRANSMITTING MEMBER, SAID PISTON BEING SUBJECT TO SIDE-THRUST LOADS DURING OPERATION, SAID DISPLACEABLE PISTON BEING PROVIDED WITH A PLURALITY OF SMALL GROOVES ARRANGED ABOUT THE OUTER SURFACE OF SAID DISPLACEABLE PISTON AND EXTENDING IN THE LENGTHWISE DIRECTION THEREOF, DUCT MEANS COMMUNICATING WITH SAID SMALL GROOVES FOR CONDUCTING PRESSURIZED FLUID FROM SAID FORCE TRANSMITTING MEMBER TO SAID SLIDING SURFACES OF THE PISTON SUBJECT TO SIDE-THRUST, SAID SMALL GROOVES POSSESSING A CROSS-SECTION WHICH DOES NOT CONSIDERABLY EXCEED THE CROSS-ECTION OF SAID DUCT MEANS AND A LENGTH AT LEAST EQUAL TO THE LENGTH OF THE RADIUS OF SAID DISPLACEABLE PISTON. 